fix: improve UX and fix admin console display issues

Miniprogram:
- Add back buttons to all custom-nav pages
- Add debug/mock buttons (BLE connect, wear check, treatment)
  controlled by __DEV__ flag (hidden in prod)

Admin console:
- Fix log page using nonexistent fields (operator_type, target_type)
  now correctly reads admin_id/user_id/action/detail from API
- Fix subscription date fields (started_at→start_time, expired_at→expire_time)
- Wire up subscription detail/extend/renew action buttons
- Wire up device detail "查看完整日志" button
- Add "创建订阅" button to subscription toolbar
- Fix subscription status mapping (2=expired, 3=cancelled)

Docs:
- Add detailed architecture docs for server, miniprogram, admin console

miniprogram/app.wxss

@@ -238,3 +238,30 @@ page {
 .mt-30 { margin-top: 30rpx; }
 .mb-20 { margin-bottom: 20rpx; }
 .mb-30 { margin-bottom: 30rpx; }
+
+.nav-back {
+  font-size: 28rpx;
+  color: rgba(255, 255, 255, 0.85);
+  padding: 8rpx 0;
+  margin-bottom: 8rpx;
+}
+
+.debug-actions {
+  margin-top: 40rpx;
+  padding: 20rpx;
+  background: #fff8e1;
+  border-radius: 12rpx;
+  border: 2rpx dashed #f5a623;
+}
+
+.debug-label {
+  font-size: 24rpx;
+  color: #f5a623;
+  margin-bottom: 16rpx;
+  font-weight: 600;
+}
+
+.debug-actions .btn-sm {
+  margin-top: 12rpx;
+  font-size: 26rpx;
+}